1. Field of the Invention
The present invention relates to a heated transfer line assembly for use in the field of liquid and gas chromatography, where the assemblies are adapted to reduce sample condensation in transfer lines leading to and away from a column in a microwave oven and to methods for making and using same.
More particularly, the present invention relates to a heated transfer line assembly for use with liquid and gas chromatography instruments, where the assembly includes a rod having an aperture therethrough adapted to receive a transfer line extending from one end of the rod and terminating near the second end of the rod. The second end of the rod is adapted to receive a lead of a column so that an amount of the column that is unheated is minimized. The assembly also includes a heating element to maintain the transfer line at a desired elevated temperature and a housing surrounding the rod over a portion of its length. The present invention also relates to methods for making and using same and to instruments including the heated transfer line assembly.
2. Description of the Related Art
Gas and liquid chromatography are physical methods for the separation, identification, and quantification of chemical compounds. These methods are used extensively for applications that include the measurement of product purity in analytical chemistry, the detection, measurement and/or determination of environmental contaminants, the detection, measurement, determination and/or characterization of natural substances, and the development of pharmaceuticals.
The fundamental methods used in gas and liquid chromatography to separate chemical constituents are similar. A sample mixture is injected into a flowing neutral carrier stream and the combination then flows through a tube or chromatographic column. The inner surface of the column is coated or the column is packed with a material called the stationary phase. As the sample mixture in the carrier stream flows through the column, the components within the stream partition between the moving phase (the stream) and the stationary phase and are retained by the stationary phase to a greater or lesser degree depending on the relative volatility (in the case of gas chromatography) or the relative solubility (in the case of liquid chromatography) of the individual components and on their respective affinities for the stationary phase. When the individual mixture components are released back into the carrier stream by the stationary phase, they are swept towards the column outlet where they are detected and measured with a detector. Different chemical compounds are retained for different times by the stationary phase. By measuring the retention times, the specific compounds in the mixture can be differentiated and/or identified. The relative concentrations of the compounds are determined by comparing the peak amplitudes measured with the detector for each compound.
The primary difference between gas and liquid chromatography is the mode of separation. In gas chromatography, the sample is volatilized and propelled down the analytical column by a moving stream of gas. In liquid chromatography, the sample is dissolved and propelled down the analytical column in a moving stream of liquid. Another difference between gas and liquid chromatography is that the columns used in liquid chromatography are generally filled or packed with the stationary phase, while those used in gas chromatography can also have the stationary phase coated or bonded to the interior wall, instead.
GC and LC measurements are facilitated by the application of heat to the chromatographic column to change its temperature. The use of a heated column oven in gas chromatographic systems greatly increases the number of compounds that can be analyzed and speeds up the time required for each analysis by increasing the volatility of higher molecular weight compounds. Heating an LC column affects the relative solubility of the mixture's components in the two phases and can enhance the separation as well as improve the repeatability of the elution times of the component chemicals.
Many methods have been described for heating chromatographic columns. The simplest and most commonly used method utilizes resistive heating elements to heat air which is in turn circulated through an insulated oven in which the column is placed. For example, U.S. Pat. No. 3,527,567 to Philyaw et al. describes a GC oven heated with resistive elements.
The resistive element heating method has several limitations. To achieve even heating of the column, a large volume of air is rapidly circulated around the chromatographic column. In addition to heating the column, the air heats the oven itself. Because the thermal mass of the oven is much larger than that of the column, the rate at which the column can be heated is commensurately reduced. A related problem is cooling time. After heating the oven to a high temperature during an analysis, it takes significantly longer to cool the oven plus the column to their initial temperature so that the next sample may be analyzed than it would to cool the column alone. Together, these limitations reduce the throughput of the chromatographic method and instrument.
Attempts to localize the resistive heat element onto the column itself so as to reduce or eliminate peripheral heating of the “oven” are described in U.S. Pat. No. 3,169,389 to Green et al., U.S. Pat. No. 3,232,093 to Burow et al., and in U.S. Pat. No. 5,005,399 to Holtzclaw et al. Each of these patents describe methods for directly wrapping or cladding the chromatographic column with a resistive heating element. Methods are also described for positioning the resulting metal clad column adjacent to a cooling source to decrease cooling times. This method of heating can be difficult to implement in practice because of uneven heating of the column due to local hot or cold spots in the resistive heating element surrounding the column. Uneven heating of the column in turn compromises the quality of the analysis.
Yet another limitation of all resistively heated chromatographic devices is that if operated improperly, they can be driven to temperatures higher than the maximum tolerated by a given column resulting in damage to or destruction of the column.
An alternative method for heating chromatographic columns is microwave heating as described in U.S. Pat. No. 4,204,423 to Jordan. Potential advantages of microwave heating are efficiency and selectivity. Suitable objects placed in a microwave oven will be heated when the oven is operated, but the temperature of the oven itself will not change. Microwave heating occurs in materials which absorb the microwave energy and convert it into heat. Current chromatographic columns are generally made of materials that do not absorb microwave energy at an appreciable rate. For example, most GC capillary columns are made of polyimide and fused silica. Consequently, such columns will not heat at an appreciable rate when placed in a microwave oven. The apparatus taught by Jordan is not practicable with these columns.
U.S. Pat. Nos. 6,514,316, 6,316,759, 6,182,504, 6,093,921, 6,029,498, and 5,939,614, incorporated herein by reference, disclose various aspect of microwave heating in GC and LC applications. These microwave heating techniques are gaining in utility. However, like all other heating system, microwave heating does present certain problems. One problem is associated with transferring material to a column in a microwave heating oven, especially in high temperature applications, where condensation in transfer lines can significantly and adversely affect GC and LC measurements.
Although the microwave heated chromatography instruments are becoming more prevalent even in high temperature applications, condensation problems still persist due to transfer line cooling in the instrument. Thus, there is a need in the art for transfer lines that reduce and/or substantially eliminate sample component condensation as a sample progresses into and out of a microwave heated column.